The present invention relates to a method for producing a fibrous pulp web, in particular a tissue or sanitary paper web.
Conventionally, a fibrous pulp suspension is dewatered in a Twin Wire Former in order to form a fibrous pulp web and then dried with the aid of a so-called through-air dryer (TAD). A pre-dewatering device with a hood and a suction roll is provided upstream of the TAD dryer. The fibrous pulp web is guided through the pre-dewatering device between exactly two machine cloths, and a hot fluid flows through the web, where the cloth between suction roll and fibrous pulp web is a felt and the cloth between fibrous pulp web and hood is a wire.
In conventional machines for the production of tissue with a TAD dryer, the fibrous pulp web is pre-dewatered first of all with the aid of vacuum to a dry content of approximately 25% and dried afterwards in the TAD dryer with large quantities of hot air. Here, air is heated by burners and conducted through the TAD hood with the aid of blowers and then through the fibrous pulp web into the TAD drum, where the water in the fibrous pulp web evaporates in this process. The differential pressure between the hood and the drum is low here. This through-air drying enables the production of particularly soft tissue.
However, the air permeability of the fibrous pulp web is a critical factor with TAD drums. If the fibrous pulp web is too damp, the hot air is unable to permeate through it. Thus, only impingement drying takes place in the first section of the TAD drum until the dry content of the fibrous pulp web is high enough for the air to pass through it. Only then is through-air drying possible. This undesirable impingement drying in the TAD dryer is a disadvantage for the moisture profile of the fibrous pulp web and, in addition, the drying efficiency drops and energy consumption increases. In order to achieve an adequate dry content, two TAD dryers are often arranged one after the other.
EP 1 397 587 B1 discloses a tissue machine that dispenses with the use of TAD dryers. Here the fibrous pulp web is pre-dried in a pre-dewatering device in which the fibrous pulp web is embedded between a wire and a felt, with hot air flowing through the fibrous pulp web at a temperature of <220° C., preferably <150° C., and an air volume flux of less than 50 m3/m2·min). Following this device, the fibrous pulp web is transferred to a Yankee dryer, where it undergoes final drying. A Yankee dryer operates according to the principle of impingement drying alone.